1. Field of the Invention
This invention relates to a disk cartridge rotatably accommodating a disk-like recording medium, such as a floppy disk, and to an equipment for molding the half of a disk cartridge from synthetic resin.
2. Description of the Prior Art
As to inventions of a 31/2 inch floppy disk, several applications have been already granted to the applicant for this invention under U.S. Pat. Nos. 4,445,155, 4,445,157, 4,510,546, 4,517,617, 4,544,977, and 4,589,105. The 31/2 inch floppy disk disclosed in those applications is shown in FIGS. 3A and 3B.
In the drawings, a pair of upper and lower halves 1 are made of synthetic resin through an injection molding, and engaged with or welded to each other to form a cartridge 2 in which a 31/2 inch floppy disk 3 made of magnetic sheet is rotatably accommodated. That is, each of the upper and lower halves 1 has a plate portion 1a and a rib portion 1b perpendicularly extending from the outer periphery of the plate portion 1a, and the free ends of the rib portions 1b of the upper and lower halves 1 are engaged with or welded to each other.
A rectangular head insertion opening 4 is provided in the center of one end of each of the plate portions 1a, and opened and closed by a shutter 5 which has a U-shaped section, and is slidably secured to the outer face of the cartridge 2. Provided in the center of the plate portion 1b of the lower half 1 is a circular opening 7 in which the core 6 of the floppy disk 3 is loosely fitted.
When above half 1 is made through the injection molding, it is well-known that molten resin is ejected from, for example, four gates 11 of about 1 mm in diameter which are disposed in the wall of a mold so as to correspond to the four corners of the plate portion 1a of the square half 1 as shown in FIG. 4, and as shown in FIG. 5A, the molten resin 14 is injected into the cavity 13 of the mold 12 in a direction perpendicular to the plate portion 1a of the half. In FIG. 5A, reference numerals 15 and 16 denote an injection head and a runner, respectively.
However, the flows of the molten resin ejected in directions of respective arrows shown in FIG. 4 from the four gates 11 collide with one another, so that various defects, such as weld lines 17, burn marks, uneven gloss 18 and the like are often found in the surface of the half 1 after molding. Further, as shown in FIG. 5B, traces 21, such as recess 19 formed by the injection head 15, and a small projection 20 formed due to tearing off the resin and the like are left in the surface of the half 1 after molding.
In addition, as the thickness of the plate portion 1a of the half 1 is less than 1 mm, and the molten resin is not only ejected from the gate 11 of about 1 mm in diameter, but also injected in the direction perpendicular to the plate portion 1a of the half 1, the time required to fill the cavity 13 with the molten resin is protracted due to the pressure loss caused by the gate 11, and as the result, much time is required to mold the half 1. If the injection pressure of the molten resin is raised, many flashes are apt to be formed in the face of the mold.